Current Switching With High Voltage Air Disconnect Or

8/2/2019 Current Switching With High Voltage Air Disconnect Or

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Current Switching with High Voltage Air

DisconnectorSalih Carsimamovic1, Zijad Bajramovic1, Miroslav Ljevak2, Meludin Veledar3, Nijaz Halilhodzic4

Abstract. In the paper are presented results of switchingovervoltages investigations, produced by operations of air

disconnector rated voltage 220 kV. Measurements of these

switching overvoltages are performed in the air-insulated

substation HPP Grabovica on River Neretva, which is an

important object for operation of electric power system of Bosnia

and Herzegovina. Investigations of operating of air disconnector

type Centre-Break were performed in order to determine

switching overvoltage levels that can lead to relay tripping in HPP

Grabovica. During operations of disconnector (synchronization or

disconnecting of generator from network) malfunctions of

signalling devices and burning of supply units of protection relays

were appeared. Also, results of computer simulations using

EMTP-ATP [1] are presented.

Key words: Switching overvoltages, air disconnector, air insulatedsubstations, secondary circuits.

I. INTRODUCTION

Switching operation in power stations and substations, high-

voltage faults and lightning cause high levels of high

frequency overvoltages that can be coupled with low voltage

secondary circuits and electronic equipment unless they are

suitably protected. The function of high-voltage air-break

disconnectors is to provide electrical isolation of one part of

the switchgear. Disconnector's standards define a negligible

current interrupting capability (0.5 A) or a voltage between

the contacts if it is not significantly changed. These values ofcurrents include the capacitive charging currents of bushing,

bus bars, connectors, very short lengths of cables and the

current of voltage instrument transformers. Disconnector's

contacts in air-insulated substations (AIS) are moving slowly

causing numerous strikes and restrikes between contacts.

When the contacts are closed, the capacitive charging current

flowing through the contacts ranges from 0.01710-3 to1.110-3 A/m for voltage levels 72.5 - 500 kV [2], dependingon the rated voltage and length of bus, which is switched.

______________________________________________

This work was supported by the Ministry of Education and Science of the

Canton Sarajevo.1

Faculty of Electrical Engineering, University of Sarajevo, Zmaja od Bosnebb, 71000 Sarajevo, Bosnia and Herzegovina (e-mail: [email protected] ;

[email protected])2 Energoinvest- Sarajevo, H.Cemerlica 2, 71000 Sarajevo, Bosnia and

Herzegovina (e-mail: [email protected])3 ABB Representation for B&H, Zmaja od Bosne bb. TPC Sentada, 71000Sarajevo, Bosnia and Herzegovina (e-mail: [email protected])4 Public Enterprise Elektroprivreda BiH, HPP on Neretva, Jaroslava Cernija 1,

88420 Jablanica, Bosnia and Herzegovina (e-mail: [email protected])_________________________________________________________

Presented at the International Conference on Power Systems

Transients (IPST05) in Montreal, Canada on June 19-23, 2005

Paper No. IPST05 - 229

Strikes and restrikes occur as soon as the dielectric strength ofthe air between contacts is exceeded by overvoltage.

The distance between contacts, the contacts geometry and

relative atmospheric condition defines the overvoltage at the

instant of strike. Every strike causes high-frequency currents

tending to equalize potentials at the contacts. When the current

is interrupted, the voltages at the source side and the loading

side will oscillate independently. The source side will follow

the power frequency while the loading side will remain at the

trapped voltage. As soon as the voltage between contacts

exceeds the dielectric strength of the air, at that distance the

restrike will occur, and so on.

Successive strikes occurring during the closing and opening

operations of the off-loaded bus by the disconnector are shownin Fig. 1 a and b, respectively.

When closing takes place, the first strike will occur at the

maximum value of the source voltage. Its values can be

positive or negative. As the time passes a series of successive

strikes will keep occurring at reduced amplitude, until the

contacts touch. The highest transient overvoltage therefore

occurs during the initial pre-arc, Fig.1 a.

When the disconnector opening, restrikes occur because of the

very small initial clearance between the contacts. At the

transient beginning, the intervals between particular strikes are

on the order of a millisecond, while just before the last strike;

the period can reach about one half of cycle at power

frequency, Fig. 1 b.

a)

b)

Fig. 1. The voltage due to the disconnector switching

a) Disconnector closing, b) Disconnector opening1-source side voltage, 2- load side voltage


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During the switching time of operations of disconnectors at

HPP Grabovica up to 500 restrikes were registered. In paper

[3] there are up to 5000 restrike registered during switching

operation of the disconnector. The maximum value of voltages

and maximum value of the wave front increasing will take

place at the maximum distance between contacts. For the

purpose of the investigation of the insulation strength and

induction of electromagnetic interferences (EMI), the most

important are the first few strikes during the closing operation

or the last few strikes during the opening operation. Each

individual strike causes a travelling wave with the basic

frequency on the order 0.5 MHz (330 kHz-600 kHz). Very fast

transient overvoltage due to the closing operation of the

disconnector at the load side of the test circuit is shown in Fig.

2.

Fig. 2. Very fast transient overvoltage due to the closing operation

Channel 1- source side voltageChannel 2-load side voltage

These high-frequency phenomena are coupled with the

secondary circuits as a result of various mechanisms. The

strongest interference is exerted by the stray capacities

between the high-voltage conductors and the grounding

system, followed by the metallic link between the grounding

system and the secondary circuits. High-frequency transient

current flowing in the grounding system generates potential

differences, every time when a strike occurs between

disconnector's contacts. In large secondary circuits, the

potential differences are in the form of longitudinal voltages

between the equipment inputs and the equipment enclosures.

Depending on the type of secondary circuits used and the way

they are laid, differential voltages may also occur. Such a

coupling mechanism has a special effect on the secondary

circuits of instrument transformers, and particularly on the

connected instruments, since these circuits are always galvanic

ally linked to the grounding system. Another factor, which

cannot be discounted, is the linking of these circuits to theprimary plant via the internal capacities of the instrument

transformers [4].

Interference levels in secondary circuits of air-insulated

substations during switching disconnectors depend on

following parameters:

-the transient voltages and currents generated by the switching

operation;

-the voltage level of the substation;

-the relative position of the source of disturbances and

susceptor;

-The nature of the grounding network;

-the cable type (shielded or unshielded);

-the way the shields are grounded.

There are two main modes of coupling secondary circuits with

primary circuits [3, 5]:

a) Electromagnetic or EM coupling, which can be split

into three sub-categories; inductive, capacitive and

radiative. The most important source of EM coupling

is the propagating current and voltage waves on bus

bars and power lines during high-voltage switching

operations by disconnectors;

b) Common impedance coupling, as a result of coupling

caused by the sharing of a lumped impedance

common to both the source and susceptor circuits.

Common mode voltages, i.e., voltages measured between

conductors and local ground, represent the main parameter

used for assessing equipment immunity. The difficulty of

comparing data comes from the fact that different authors

performed measurements at different places (some

measurements were made at the closest point to the

disconnector being operated whereas others made

measurements in the vicinity of the auxiliary equipment, i.e. in

the relay room). Little information is available about thegrounding practice of the neutral conductor in CT or VT

circuits, the quality and grounding of the sable shields as well

as how the measurements have been performed. Therefore, the

measured levels have to be analyzed very carefully before

comparison and drawing any conclusions [5].

Results of up to date measured common mode voltages at

secondary circuits of CVT, CT and VT are presented in the

paper [5]. There are maximum levels of the common mode

voltages ranging from 100 Vpeak up to 2.5 kVpeak in the shields

of the secondary circuits cables of the CT and VT. Results

show that measured values of the common mode voltages at

CT/CV secondary circuits, 220 kV ratings, range from

Ucm=0.32 kVpeak [6] up to Ucm=0.85 kVpeak [7]. Resultsshown in paper [3] are for measured common mode voltages

from 3-4 kV during switching operation by disconnector in 150

kV switchgear up to 6-10 kV at 400 kV switchgear.

II. RESULTS OF EXPERIMENTAL MEASUREMENTS ON

SITE

The last ten years of extensive analysis of disconnector and

circuit breakers generated EMI measurements that have

confirmed that disconnector operation with off-loaded busbar

is the most important and typical source of interference in

secondary circuits of substations. Measurements of switching

overvoltages generated during disconnector operation in the

air insulated substation HPP Grabovica on the river Neretva

were performed. HPP Grabovica is an important object for

operating of electric power system of Bosnia and

Herzegovina. Investigations of operating of air disconnector

type Centre-Break were performed in order to determine

switching overvoltage levels that can lead to relay tripping in

HPP Grabovica [8]. During operations of disconnector

(synchronization or disconnecting of generator from network)

malfunctions of signalling devices and burning of supply units

of protection relays were appeared. Malfunctioning of

auxiliary circuits were manifested by tripping relay of


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differential protection of the generator, phase '4'- signalization

on relay box 'ZB I' and signalling fire in 35 kV control panel.

At the same time sparking between primary terminals of the

current transformer (CT) was occurred. Malfunctioning of

signalling circuits were lower (not eliminated) with installing

shielded cables.

Also, independent of switching operation of air insulated

disconnectors, during synchronization of generator AG1 on

network, its happened that one of the pole of 220 kV circuit

breaker failures. In this case generator AG1 worked in motor

regime. Because of that, HPP Grabovica plans to install circuit

breakers on generators voltage (10,5 kV) [9].

The field tests were performed at the test circuit at HPP

Grabovica, Fig. 3.

Fig. 3. The considered test circuit

VT-voltage transformer (220/3/0.1/3/0.1/3 kV), CT-current transformer(200/1/1 A), CVD-capacitive voltage divider, CB-circuit breaker with two

interrupting chambers and parallel capacitors (SF6 220 kV, 1600 A), Dc-

disconnector (220 kV, 1250 A), MOSA-metal oxide surge arrester (Ur=199,5

kV, 10 kA), PT-power transformer (64 MVA, 242/10,55% kV, YD5), AG1-generator 1 (64 MVA, 10,55% kV)

The recorded wave shape of the overvoltage at the load side is

shown in Fig. 4. The overvoltage factors at busbar, k, wererecorded up to 1.16 p.u. with the dominant frequency of

considered transient fd equal to 0.536 MHz. Common mode

voltages, Ucm, at VT were up to 708 Vpeak, with dominant

frequency equal to 1.31 MHz.

Fig. 4. Waveshape of the overvoltage

Channel 1-voltage at CVD; ch 1 (2.5 V/div), probe 1x100, ratio 455

Channel 2-voltages at secondary of VT; ch 2 (5 V/div), probe 1x100

III. MODELING OF THE TEST CIRCUIT

Computer simulations were performed on the model of test

circuit containing elements drawn in Fig. 5. Overvoltages at

busbars were calculated during disconnector closingoperations, for the same substation layout on which

measurements were carried out.

CB

VT

CVD

arc

PTstray

connection

wire

connection

tubeDc

II

IIII

II

busbars 220 kV

model of

network 220 kV

CIIIIIII

Ccb

CT+MOSA

C

Fig. 5. Model of the test circuit

Arc-4 ; stray-200 pF; connection tube Z=370 ; CVD-R=300 , C=1 nF;VT-500 pF; CB-2 capacitors, each C2 nF, (capacitance of open contacts,each C20 pF), Ccb=100 pF; CT-500 pF; MOSA-100 pF; connection wireZ=440 ; PT-3.5 nF

The waveshape of simulated overvoltage surge at load side is

given in Fig. 6. The difference between magnitudes of

measured and simulated overvoltages is 5 %. The dominant

frequency of simulated overvoltage is 0.620 MHz. Comparison

between results of measured and calculated overvoltagescertified a good agreement of obtained values.

245 kV4400 pF

CVD

Relay room

AG1

1

MOSA

M

HPP GRABOVICA LAYOUT

OL TO RP JABLANICA M

M0.2 mH1250 A

Dc

VT

CB

CT

PT

BUS BARS 220kV 3~50 Hz, 1250 A

1nF300

0.44F

2nF

2nF


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(f ile grabo-kontrola.pl4; x-var t) v :XX0007

2.858 2.860 2.862 2.864 2.866 2.868 2.870 2.872[ms]-200

-150

-100

-50

0

50

100

[kV]

Fig. 6. Waveshape of simulated overvoltage surge

When the Capacitive Voltage Divider (CVD) was excluded,

there were higher values of calculated overvoltages (15%

higher on amplitude and 6 % on frequency). Capacitive divider

due to primary resistor equal to 300 and primary capacitanceequal to 1 nF influences on overvoltage at the same

measurement point causing attenuation and damping oftransient overvoltrages.

In order to reduce EMI in secondary circuits the best way is to

reduce sources of interference emission during switching of air

insulated disconnector. One of the ways of reducing is to

install disconnecting circuit breakers. Substation disconnectors

isolate circuit breakers from rest of the system during

maintenance and repair. The maintenance requirements for

modern SF6 high voltage circuit breakers are lower than

maintenance demands made on disconnectors, which means

one of reasons for disconnectors removed. Installing

disconnecting circuit breaker there are no needs for switching

operation of disconnectors. With disconnecting circuit

breakers it is still possible to isolate the line, but lowmaintenance requirements means it is no longer necessary to

isolate the circuit breaker. The disconnecting breaker had to be

designed to safety lock in the open position, and to meet all

voltage withstanding capabilities and safety requirements of

disconnectors.

Another way of reducing sources of interference emission is to

install circuit breaker without parallel capacitors to contacts.

This suggestion is based on analyses performed on three

circuit models:

a) model of CB with two breaking chambers and paralelcapacitors and VT on netvork side of CB;

b) model of CB with two breaking chambers and withoutparalel capacitors and VT on netvork side of CB

c) model of CB with two breaking chambers and withoutparalel capacitors and VT on generator side of CB

Magnitudes of simulated overvoltages are presented in Table

I. Voltages are measured in point of connection of VT, CT and

PT.

TABLE I

MAGNITUDES OF SIMULATED OVERVOLTAGES

Connection point

Circuit model

VT CT PT

a) model of CB with

two breaking chambersand paralel capacitors

and VT on netvork side

of CB

169 kVf=620 kHz

47 kVf=1,1 MHz

51 kVf=620 kHz

b) model of CB withtwo breaking chambers

and without paralel

capacitors and VT on

netvork side of CB

177 kV

f=900 kHz

560 V

f=1,4 MHz

165 V

f=1,4 MHz

c) model of CB with

two breaking chambers

and without paralel

capacitors and VT on

generator side of CB

320 Vf=1,1 MHz

320 Vf=1,1 MHz

160 Vf=1,1 MHz

Overvoltages on generator side of 220 kV CB during

switching of disconnectors could be up to 320 V in the case of

installing instrument voltage transformer (VT) on generator

side of CB without parallel capacitors (near instrument current

transformer CT). This case causes installing of circuit breakerat generators voltage (10,5 kV) for synchronization of

generator to network (better conditions for synchronization).

This solution of installing circuit breakers on generators

voltage resulted from problems have occurred during

synchronization of generatror with current 220 kV CB.

IV. CONCLUSION

Switching overvoltages due to disconnector operations have

been analysed on the existing 220 kV AIS on HPP Grabovica.

Measurements and calculations were conducted on the

characteristic points in AIS, in order to determine the level of

the EMI.

The result of measurements has shown that high frequency

voltages on busbars occur with amplitudes up to 1.16 p.u. (233

kVpeak) and the dominant frequencies up to 0.6 MHz.

The difference between magnitudes of measured and calculated

overvoltages is 5 % and 15.6 % on frequency.

Measured common mode voltages at secondary circuits were

from 430 V up to 708 V.

CVD influences on overvoltages at the same measurement

point on busbars causing attenuation and damping of transient

overvoltages.

Comparison of the transient computer simulations with field

measurements showed that calculations could be used for

assessment of the transient overvoltages due to disconnector

switching.In order to reduce EMI in secondary circuits, it is suggested to

install switching modules and disconnecting circuit breakers

[10] or to install circuit breakers without parallel capacitors to

contacts.


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V. REFERENCES

[1]EMTP-ATP, European EMTP-ATP Users Group e.V.

[2] D.F.Peelo, J.H.Sawada, B.R.Sunga, R.P.P.Smeets, J.G.Krone, L.Van Der

Sluis, 'Current interruption with high voltage air-break disconnectors', CIGRE

2004, paper A3-301[3] Guide on EMC in Power Plants and Substations, CIGRE WG 36.04, Dec.

1997

[4] H.Remde, H.Schwarz, 'Transient overvoltages in CT and VT secondary

circuits in high-voltage substations',ABB Review 1/91[5] C.Imposimato, J.Hoeffelman, A.Eriksson, W.H.Siew, P.H.Pretorius,

P.S.Wong, EMI Characterization of HVAC Substations-Updated Data and

Influence on Immunity Assessment, CIGRE 2002, paper on behalf of

CIGRE/CIRED WG 36/S2-04

[6] R.M.Naumov, P.L.Vukelja, 'Experimental Investigations of Transient

Overvoltages in Secondary Circuits of 400 and 220 kV High VoltageSubstations', CIGRE SC36 Symposium, Lausanne, paper 500-05, 1993

[7] R.J.Gavazza, C.M.Wiggins, 'Reduction of Interference on Substation Low

Voltage Wiring', IEEE Trans. On Power Delivery, Vol. 11, No.3, 1317-1329,July 1996

[8] 'Overvoltages in primary and secondary circuits in HPP Grabovica due to

disconnector switching', Faculty of Electrical Engineering, Report No.L1410010/04, Sarajevo, 2004

[9]D.Braun, L.Widenhorn and J.Ischi, Impact of the Electrical Layout on the

Availability of a Power Plant, 11-th CEPSI Conference, 21-25 October

1996, Kuala Lumpur, Malaysia[10] C-E. Slver, H-E. Olovsson, W.Lord, P.Neorberg and J.Lundquist,

Innovative substations with availability using switching modules and

disconnecting circuit breakers, CIGRE 2000, paper 23-102

Current Switching With High Voltage Air Disconnect Or

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